1. Field of the Invention
The present invention relates to a nuclear magnetic resonance (NMR) imaging for obtaining tomographic information and spectroscopic information of an object to be examined, and more particularly, to a measurement a speed of a flowing target object such as blood by utilizing the NMR imaging.
2. Description of the Background Art
An NMR imaging has been known as a method for obtaining tomographic information and spectroscopic information of an object to be examined non-invasively, and it is known conventionally that this NMR imaging can be utilized in measuring a speed of a flowing target object such as blood or cerebrospinal fluid.
Namely, when an imaging target blood vessel is placed along a Z axis, a pre-saturation pulse is applied to a saturation plane located in a direction perpendicularly intersecting this blood vessel, such as a direction along an X axis, in order to selectively saturate only those spin components of the blood flowing in this blood vessel which are located within this saturation plane. For the pre-saturation pulse, 90 degree pulse is normally used.
Then, the usual NMR imaging for taking an image of a slicing plane containing this blood vessel, such as a slicing plane along a Y axis, is carried out by the application of appropriate RF pulses and gradient magnetic fields according to a desired imaging pulse sequence such as a spin echo (SE) sequence or a field echo (FE) sequence, and the NMR echo signals are collected after an elapse of an echo time TE, such that the NMR image of the blood vessel can be reconstructed from the collected NMR echo signals. In the reconstructed NMR image of the blood vessel, the spin components saturated by the pre-saturation pulse appear as a dark region called Tag portion.
Here, the Tag portion has flowed through the blood vessel for a distance d from the position of the saturation plane during the time period T between the application of the pre-saturation pulse and the detection of the NMR echo signals, so that the Tag portion appears to be displaced from the saturation plane by the distance d in the reconstructed NMR image. Consequently, by measuring this distance d between the position of the saturation plane and the Tag portion on the reconstructed NMR image, it becomes possible to determine the speed of the blood flow as V=d/T. Usually, this type of measurement is repeated for measurements are averaged in order to remove the temporal fluctuation factor.
However, in this conventional method of measuring the speed of the blood flow, when the speed of the blood flow becomes faster than a certain limit speed, the shape of the Tag portion is disturbed by the turbulence in the blood flowing in vicinity of the Tag portion, such that the moving distance d of the Tag portion cannot be determined accurately and consequently the speed of the blood flow cannot be determined accurately. As a consequence, it has been difficult conventionally to accurately measure the speed of the fast blood flow such as those in the major blood vessels.